1. Field of the Invention
The present invention relates to a low-temperature sintering ceramic material co-sinterable with a low-melting point metal material such as silver or copper, and a ceramic substrate made of such a low-temperature sintering ceramic material, and further relates to a multilayer ceramic substrate.
2. Description of the Related Art
A low-temperature sintering ceramic (LTCC: Low Temperature Cofired Ceramic) material is often used as a substrate material for a high frequency module in an information communication terminal or the like because it can be co-fired with a low-melting point metal material such as silver or copper having a small specific resistance, and can form a multilayer ceramic substrate having excellent high frequency characteristics.
As a low-temperature sintering ceramic material, a so-called glass ceramic composite system in which a B2O3-SiO2-based glass material is mixed into a ceramic material such as Al2O3 is generally known, however, in this system, relatively expensive glass needs to be used as a starting material, and boron that is likely to volatilize during firing is contained, so that the composition of an obtained substrate tends to vary, and also management thereof is complicated because a special sheath or sagger should be used.
For addressing this, low-temperature sintering ceramic materials described, for example, in Japanese Patent Application Laid-open No. 2002-173362 and Japanese Patent Application Laid-open No. 2008-044829 have been proposed. The low-temperature sintering ceramic materials described in these documents will not encounter the problems as described above because they do not use glass as a starting material, and are non-glass based low-temperature sintering ceramic materials not containing boron.
However, since a ceramic substrate obtained from the low-temperature sintering ceramic materials described in these documents has a bending strength of about 150 to 200 MPa, the strength of the substrate itself can be insufficient depending on the use application or the bonding strength with an external conductor film formed on surface of the substrate can be insufficient.